IN 2003, while geneticist Svante Pääbo was visiting Novosibirsk, Russia’s third-largest city, he decided to look in on a famous experiment run by the Institute of Cytology and Genetics, which is based in the city. Fifty years ago, the then head of the IC&G, geneticist Dmitry Belyaev, had begun breeding silver foxes to see how easily they could be tamed. What Pääbo didn’t know, though, is that Belyaev had also set up another experiment in the 1970s involving rats. This time, one line of rats was selected for tameness and another selected for aggression.

When Pääbo saw them, he was stunned. After just 30 years of selection, the IC&G researchers had fashioned two populations that could hardly be more different. “I could take the tame ones out of the cage with my bare hands. They would creep under my shirt and seemed to actually seek and enjoy contact,” recalls Pääbo. “The aggressive animals were so aggressive I got the feeling that 10 or 20 of them would probably kill me if they got out of the cages.”

“The aggressive rats were so aggressive I got the feeling that 10 or 20 of them would kill me if they got out of the cage”

Here was a great opportunity to uncover the genetic changes responsible for the …